Plastic wastes have been disposed typically by incineration or landfilling. However, formation and discharge of harmful by-products produced as a result of incineration, reduction of landfill space and environmental pollution caused by unauthorized dumping have become significant. With an increasing public concern on disposal of plastic wastes, research and development on biodegradable plastics such as aliphatic polyesters have been increasingly carried out. Such biodegradable plastics are decomposed by the action enzymes or microorganisms. Among the biodegradable aliphatic polyesters, poly(lactic acid)s have been aggressively investigated and developed.
The poly(lactic acid)s are polymers produced by preparing lactic acid typically from a starch derived from corns or potatoes and subjecting lactic acid to chemical synthesis. The poly(lactic acid)s exhibit superior mechanical properties, heat resistance and transparency among aliphatic polyesters. Research and development to provide formed plastics of poly(lactic acid)s, such as films, sheets, tapes, fibers, ropes, nonwoven fabrics and packages, have been increasingly carried out. The poly(lactic acid)s as intact, however, have insufficient flexibility, and plasticizers are used to increase the flexibility of poly(lactic acid)s for use as, for example, packaging wrap films, stretch films and agricultural mulch films.
Japanese Unexamined Patent Application Publication No. 4-335060 discloses a technique of using a plasticizer such as a phthalic ester which is generally used in vinyl chloride polymers. When a regular plasticizer such as a phthalic ester is added to give flexibility to a poly(lactic acid), the resulting formed plastics exhibits flexibility immediately after addition. However, if the formed plastics are left in normal atmosphere (atmosphere of the air) particularly at high temperatures over a long time, the formed plastics exhibits remarkably reduced flexibility and deteriorated transparency because the plasticizer in the formed plastics evaporates and bleeds out. In addition, when a regular plasticizer is added to give flexibility to a poly(lactic acid), the formed plastics exhibits remarkably reduced flexibility and decreased transparency in water, particularly in hot water, because the plasticizer is extracted.
U.S. Pat. No. 5,180,765 and No. 5,076,983, and Japanese Unexamined Patent Application Publication No. 6-306264 each disclose a technique of using lactic acid, a linear lactic acid oligomer or a cyclic lactic acid oligomer as a plasticizer. A poly(lactic acid) containing a considerable amount of lactic acid, a linear lactic acid oligomer or a cyclic lactic acid oligomer, however, exhibits poor thermal stability upon forming and is easily hydrolyzed under regular conditions for use. Formed plastics such as a film prepared from such a composition exhibits decreased strength in a relatively short time and is significantly insufficient in practical utility as formed plastics.
Japanese Unexamined Patent Application Publication No. 8-199052 discloses a technique on a composition including a poly(lactic acid)/poly(alkylene ether) copolymer, and a plasticizer mainly comprising a poly(alkylene ether). According to this technique, however, the evaporation, migration and extraction (bleedout) of the plasticizer are not sufficiently controlled, although the composition exhibits flexibility at practical level.
Japanese Unexamined Patent Application Publication No. 8-253665 discloses a composition containing a polymer mainly comprising lactic acid, and a block copolymer between a poly(alkylene ether) and a poly(lactic acid). This technique is achieved in order to impart antistatic property, and the block copolymer containing the poly(lactic acid) component is added as an antistatic agent. The publication indicates that the poly(lactic acid) component serves to increase chemical affinity of the block copolymer for a base material (matrix) to thereby make the block copolymer finely dispersed. The publication, however, gives neither indication on the other actions of the poly(lactic acid) component nor concrete suggestion on the molecular weight thereof. We have double-checked the descriptions of Japanese Unexamined Patent Application Publication No. 8-253665 from the viewpoints of the flexibility of the composition, and control (prevention) of the evaporation, migration and extraction (bleedout) of the additive (plasticizer) and losing transparency upon heating as formed plastics. But we have found that these properties are insufficient.
As is described above, attempts have been made to give the flexibility to a poly(lactic acid) by adding a plasticizer. No technique, however, has been achieved to impart sufficient flexibility to a poly(lactic acid) and to control the evaporation, migration and extraction (bleedout) of the plasticizer and losing transparency upon heating in use as formed plastics.
Attempts have also been made to provide techniques for imparting typically flexibility to a poly(lactic acid) film having transparency and heat resistance to thereby use it as a trash bag or agricultural film, and techniques for imparting typically flexibility and adhesion to a poly(lactic acid) film to thereby use it as a packaging wrap film.
For use as packaging wrap films, Japanese Unexamined Patent Application Publication No. 2000-26623, for example, discloses a stretched film comprising a composition containing a liquid additive and a resin mainly containing an aliphatic lactic acid-based polyester. We have made an actual attempt to form a stretched film in accordance with the examples described in Japanese Unexamined Patent Publication No. 2000-26623. The resulting stretched film exhibits flexibility, heat-resistance and transparence at certain levels as a packaging wrap film for food packaging only immediately after film formation. The film, however, loses its flexibility and adhesion and lacks practical utility after use or storage at room temperature for about several weeks, since the liquid additive readily evaporates, bleeds out and attaches to a substance to be wrapped.
As is described above, no packaging wrap film comprising a poly(lactic acid) polymer composition being excellent in flexibility, transparency, heat resistance and adhesion has yet been realized.